This invention relates to an apparatus and system for supplying gaseous fluid under pressure to a vessel containing material. More particularly, the invention relates to an apparatus for supplying gaseous fluid under pressure to a apparatus for blending particulate material.
Prior to the present invention, blenders for particulate material such as plastic pellets are known. The blenders of the type to which the present invention relate include a vessel which may have a vertically oriented lift pipe mounted in the center of the vessel which serves as a means for circulating material in the bottom of the vessel to the top of the vessel. This is accomplished by supplying gaseous fluid or air under pressure to the bottom of the vessel to be directed up through the vertical lift pipe. This air under pressure entrains material in the bottom vessel conveys it up through the lift pipe and the material is discharged from the lift pipe in a geyser-like manner into the top of the vessel to thereby circulate material through the vessel. In order to carry out this circulation of material through the lift pipe, it is necessary to employ a given gas velocity at a given pressure. However, during the start-up phase of the blending of material, the vessel may contain a quantity of material which is at rest and the lift pipe may be partially filled with material. In order to commence the circulation of material through the lift pipe and hence blending of material. It is necessary to overcome the static head of material which may be contained within the lift pipe and/or within the vessel surrounding the inlet to the lift pipe.
In order to provide the necessary pressure, systems usually include a rotary positive displacement blower or other medium pressure gas supply device supplying gas under pressure in the range of about 4 to 6 pounds per square inch with gas velocities on the order of 2500 to 3500 feet per minute. This pressure is higher than that which is required during the normal operation of the system after start-up. During normal operation, air under pressure in the range of about 11/2 to 21/2 pounds per square inch is required. Such pressure and the required volume can be supplied by a lower pressure centrifugal blower or high pressure fan.
A centrifugal blower or high pressure fan has a lower initial capital cost and lower maintenance requirements than a rotary positive displacement blower. With a centrifugal blower, the motor will not be overloaded when the discharge air flow is reduced to zero and does not require a safety release valve at that condition. Further, a centrifugal blower or a high pressure fan is much quieter than a rotary positive displacement blowerr and therefore silencers are not required during operation. For these reasons, it would be desirable to be able to use a centrifugal blower or high pressure fan in a blending system for particulate material rather than a rotary positive displacement blower, but there are certain disadvantages. Unfortunately, the basic performance characteristics of a centrifugal blower or high pressure fan lack the high pressure surge capabilities to break through an initial head of material in the vessel such as during start-up or blender restart.